Radiotelephones generally refer to communications terminals which provide a wireless communications link to one or more other communications terminals. Radiotelephones may be used in a variety of different applications, including cellular telephone, land-mobile (e.g., police and fire departments), and satellite communications systems.
Radiotelephones typically include an antenna for transmitting and/or receiving wireless communications signals. Historically, monopole and dipole antennas have perhaps been most widely employed in various radiotelephone applications, due to their simplicity, wideband response, broad radiation pattern, and low cost.
However, radiotelephones and other wireless communications devices are undergoing miniaturization. Indeed, many contemporary radiotelephones are less than 11-12 centimeters in length. As a result, antennas utilized by radiotelephones have also undergone miniaturization. In addition, it is becoming desirable for radiotelephones to be able to operate within widely separated frequency bands in order to utilize more than one communications system. For example, GSM (Global System for Mobile communication) is a digital mobile telephone system that typically operates at a low frequency band, such as between 880 MHz and 960 MHz. DCS (Digital Communications System) is a digital mobile telephone system that typically operates at high frequency bands between 1710 MHz and 1880 MHz.
Small radiotelephone antennas typically operate within narrow frequency bands. As a result, it can be difficult for conventional radiotelephone antennas to operate over widely separated frequency bands. Furthermore, as radiotelephone antennas become smaller, the frequency bands within which they can operate typically become narrower.
Helix antennas are increasingly being utilized in handheld radiotelephones that operate within multiple frequency bands. Helix antennas typically include a conducting member wound in a helical pattern. As the radiating element of a helix antenna is wound about an axis, the axial length of the helix antenna can be considerably less than the length of a comparable monopole antenna. Thus, helix antennas may often be employed where the length of a monopole antenna is prohibitive.
FIG. 1 illustrates a conventional helix antenna 5 configured for dual frequency band operation. As shown in FIG. 1, the antenna 5 generally includes an antenna feed structure 6, a radiating element 7, and a parasitic element 8. The radiating element 7 and parasitic element 8 are housed within a plastic tube or radome 9 with an end cap 10. Unfortunately, helix antennas can be somewhat complex to manufacture, particularly with regard to positioning of the radiating and parasitic elements 7, 8.
Branch antennas are also being utilized in handheld radiotelephones that operate within multiple frequency bands. Branch antennas typically include a pair of conductive traces disposed on a substrate that serve as radiating elements and that diverge from a single feed point. FIG. 2 illustrates a conventional branch antenna 15 configured for dual frequency band operation. As shown in FIG. 2, the antenna 15 generally includes a flat substrate 16 having a pair of meandering radiating elements 17a, 17b disposed thereon. The meandering radiating elements 17a, 17b diverge from a feed point 18 that electrically connects the antenna 15 to RF circuitry within a radiotelephone. Each of the meandering radiating elements 17a, 17b is configured to resonate within a respective frequency band.
Unfortunately, branch antennas may transmit and receive electrical signals within a band of frequencies that are too narrow for radiotelephone operation. Furthermore, in order to decrease the size of a branch antenna, it is typically necessary to compress the meandering pattern of each radiating element. Unfortunately, as the meandering pattern of a radiating element becomes more compressed, the frequency band within which the radiating element can operate typically becomes more narrow.
Thus, in light of the above-mentioned demand for multiple frequency band radiotelephones and the problems with conventional antennas for such radiotelephones, a need exists for small radiotelephone antennas that are capable of operating in multiple widely separated frequency bands.